Optical transmitter with forward controlled peltier device

ABSTRACT

The present invention provides an optical transmitter with a forward controlled Peltier driver. The temperature sensor generates a monitoring signal Sm. The comparator compares this monitored signal Sm with the target temperature signal St to generated the temperature difference signal Sn. The limiter restricts this difference signal Sn between the positive and negative maximum of the periodic signal Sw to generate the limiting signal Sa. The second comparator compares this limiting signal Sa with the periodic signal Sw to generate the control signal. The PWM signal generator, responding to this limiting signal Sa, generates a plurality of PWM signals, Sg 1  to Sg 4.  The H-bridge including four transistors and two inductors, responding to this PWM signals, controls the direction of the current flowing in the Peltier device. The magnitude of the current can be determined by the duty ratio of the PWM signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical transmitter, in particular, relates to an optical transmitter with forward controlled Peltier device.

2. Related Prior Art

It is well known that an optical transceiver with a Peltier device to control a temperature of the semiconductor light-emitting device in order to adjust the oscillation wavelength of the light-emitting device. Such optical transmitter sometimes controls the Peltier device with a PWM signal. FIG. 7 shows one example of the optical transmitter presented from MAXIM. This optical transmitter provides a resistor R10 in series to the Peltier device, and a voltage drop generated between terminals of this resistor R10 is monitored by the driver 82. The driver 82 controls the current flowing in the Peltier device based on the voltage drop by the resistor R10.

However, the conventional optical transmitter configures a feedback loop such that the current flowing in the Peltier device is controlled based on the monitoring signal. Accordingly, the phase compensation is necessary within the feedback loop to control the Peltier device with a large time constant, which makes the controller complicated. Moreover, the monitoring resistor R10 must be inserted in the current path to the Peltier device, which accompanies the power loss.

SUMMARY OF THE INVENTION

Therefore, an aspect of the present invention is to provide a controller of the Peltier device without sensing the current flowing in the Peltier device.

An optical transmitter of the present invention, which provides a forward controlled Peltier device without any current sensing resistor connected in series to the Peltier device, comprises a light-emitting device, a temperature sensor, and a controller. A temperature of the light-emitting device is controlled by the Peltier device. The temperature sensor monitors the temperature of the light-emitting device and generates a temperature-monitored signal to the controller.

The controller, receiving the monitored signal, compares this monitored signal with a target signal corresponding to a target temperature of the light-emitting device by a first comparator, and outputs a temperature difference signal based on this comparison. The temperature difference signal denotes a temperature difference between the practical temperature of the light-emitting device, which is monitored by the temperature sensor, and the target temperature. The temperature difference signal is sent to a limiter within the controller. The limiter cramps the temperature difference signal between a preset maximum value and a preset minimum value. That is, when the temperature difference signal is greater than the preset maximum value, the limiter outputs the preset maximum value as the temperature difference signal. On the other hand, when the temperature difference signal is smaller than the preset minimum value, the limiter outputs the preset minimum value as the temperature difference signal.

The controller may further include a second comparator. The second comparator receives the temperature difference signal from the limiter and a periodic signal with a triangular shape, and slices this periodic signal by the temperature difference signal to generate a control signal with a pulsed shape. The periodic signal has a positive peak value, which is greater than the preset maximum value, and a negative peak value, which is smaller than the preset minimum value. Since the limiter cramps the temperature difference signal between the preset maximum and minimum values, the second comparator always slices the periodic signal and always generates a control signal with the pulsed waveform. The pulse width of the control signal depends on the level of the temperature difference signal.

The optical transmitter of the present invention may further provide a switching ladder. This switching ladder may comprise a pair of two switches. Each switch includes a pair of switching devices operated in complementary. Therefore, the switching ladder includes four switching devices, two of which may turn on and the other two of which may turn off in the same time, and these switching devices are turned on or turned off by the control signal. The Peltier device may be connected between switches through a low pass filter including an inductor and a capacitor. The current direction flowing in the Peltier device may be altered by switching the pair of two switches in complementary.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of the optical transmitter according to the present invention;

FIG. 2 is a circuit diagram of the limiter;

from FIG. 3A to FIG. 3E show, when the temperature difference signal is lower than the upper limit and greater than the lower limit, the upper and lower limit signals, the temperature difference signal, the limit signal, the periodic signal, and the control signal, respectively;

from FIG. 4A to FIG. 4E show, when the temperature difference signal is greater than the upper limit, the upper and lower limit signals, the temperature difference signal, the limit signal, the periodic signal, and the control signal, respectively;

from FIG. 5A to FIG. 5E show, when the temperature difference signal is smaller than the lower limit, the upper and lower limit signals, the temperature difference signal, the limit signal, the periodic signal, and the control signal, respectively;

FIG. 6 explains the operation of the H-bridge and the Peltier device; and

FIG. 7 shows a conventional optical transmitter.

DESCRIPTION OF PREFERRED EMBODIMENTS

Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. If possible, the same numeral or the same symbol will indicate the same element without overlapping explanations.

FIG. 1 is a block diagram of the optical transmitter according to the present invention. The optical transmitter 1 comprises a semiconductor light-emitting device 2, a Peltier device 4, a temperature sensor 6, and a Peltier driver 8. The light-emitting device 2 may be a semiconductor laser diode.

The Peltier device 4 controls a temperature of the light-emitting device 2. In practical, the Peltier device 4, a current flowing therein by the magnitude and the direction responding to a signal output from the Peltier driver 8, raises and falls the temperature of the light-emitting device 2.

The temperature sensor 6 generates a monitored signal Sm corresponding to the temperature of the light-emitting device 2. The temperature sensor 6 is one type of a resistor with resistance thereof largely varying with temperatures. One terminal of the temperature sensor 6 connects with a load resistor R1 in series, while the other terminal of the load resistor R1 connects to the power line Pa. The monitored signal Sm is output from the node connecting the temperature sensor 6 to the load resistor R1. The temperature sensor 6 may be a thermistor.

The Peltier driver 8 drives the Peltier device 4 with a PWM (Pulse Width Modulation) signal. The Peltier driver 8 includes a controller 10, a PWM signal generator 12, and the switching ladder 14.

The controller 10 generates a control signal Sc for the PWM signal generator 12. The controller 10 comprises the first comparator 16, the limiter 18, and the second comparator 20. The first comparator 16 compares the monitored signal Sm output from the temperature sensor 6 with the target temperature signal St from the input 22, and generates a temperature difference signal Sn. The target temperature signal St corresponds to the target temperature of the light-emitting device 2. The limiter 18 generates a limiter signal Sa that restricts the range of the temperature difference signal Sn.

FIG. 2 is a circuit diagram of the limiter 18. The limiter 18 includes a buffer amplifier 26, an inverting amplifier 28, and two diodes, 30 and 32.

The buffer amplifier 26 generates the upper limit signal Su by receiving the limiter signal Sp at the input 24, which determines maximum and minimum of the duty ratio for the PWM signal, from Sg1 to Sg4. The upper limit Su is smaller than the positive maximum of the periodic signal Sw (see FIG. 3D).

The inverting amplifier 28 receives a reference Pb and the upper limit Su from the buffer amplifier 26. The reference Pb may be determined based on an average of the periodic signal Sw. The inverting amplifier 28 generates the lower limit Sd by inverting the upper limit Su. The lower limit Sd is greater than the negative maximum of the periodic signal Sw (see FIG. 3D).

The cathode of the first diode 30 is connected to the buffer amplifier 26, while the another thereof connected to the input to which the temperature difference signal Sn is input. The anode of the second diode 32 is connected to the output of the inverting amplifier 28, while the cathode thereof is connected to the input receiving the temperature difference signal Sn. Thus, the cathode of the first diode 30 receives the upper limit Su, while the anode of the second diode 32 receives the lower limit Sd. The limiter 18 outputs the limit signal Sa.

Referring to FIG. 1 again, the second comparator 20, by receiving the limit signal Sa from the limiter 18 and the periodic signal Sw from the input 34, outputs the control signal Sc to the PWM signal generator 12. The periodic signal Sw may be a triangular wave, while the control signal becomes a rectangular wave.

The PWM signal generator 12, responding the control signal Sc output from the controller 10, generates PWM signals, Sg1 to Sg4. The duty ratio of the control signal Sc determines the pulse width of the PWM signals, Sg1 to Sg4.

The switching ladder 14, which is the so-called as the H-bridge, drives the Peltier device 4 by responding to the PWM signals, Sg1 to Sg4. The H-bridge may include, for example, two p-channel FETs (Field Effect Transistor), 41 and 42, two n-channel FETs, 43 and 44, two capacitors, 51 and 52, and two inductors, 61 and 62. These FETs, 41 to 44 operate as a switching device, and the left row of FETs, 41 and 43, forms one switches, while the right row of FETS, 42 and 44, forms another switches. These switches operate in complementary as described below. Sources of p-type FETs, 41 and 42 are connected to the power line Pc. The PWM signals, Sg1 to Sg4, lead to the gate of the FETS, 41 to 44, respectively. The drain of the first FET 41 leads to the drain of the third FET 43, while the drain of the second FET 42 leads to the drain of the fourth FET 44. The first inductor 61 connects the drain of the first and third FETs, 41 and 43, to the one terminal 4 a of the Peltier device 4, while the second inductor connects the drain of the second and fourth FETs, 42 and 44, to the other terminal 4 b of the Peltier device 4.

Next, the operation of the optical transmitter 1 will be described. First, the temperature sensor 6 generates the monitored signal Sm responding to the temperature of the light-emitting device 2. The first comparator 16 compares this monitored signal Sm with the target temperature signal St, and generates the temperature difference signal Sn that corresponds to a difference between the monitored signal Sm and the target signal St.

The temperature difference signal Sn is input to the limiter 18. The buffer amplifier 26 of the limiter 18 outputs the upper limit Su based on the limiter signal Sp. The inverting amplifier 28 outputs the lower limit Sd based on the limiter signal Sp input at the terminal 24. When the temperature difference signal is between the upper limit Su and the lower limit Sd, the first and second diodes, 30 and 32, are biased in reverse. In this case, the limiter 18 passes the temperature difference signal Sn and outputs as the limiter signal Sa. When the temperature difference signal Sn is greater than the upper limits Su, the first diode 30 is biased in forward and the second diode is biased in reveres. Accordingly, the limiter outputs, not passing the temperature difference signal Sn, the limiter signal Sa with an upper limit value Vu. When the temperature difference signal Sn is smaller than the lower limit Sd, the first diode is biased in reverse and the second diode is biased in forward. Therefore, the limiter 18 outputs the limiter signal Sa with a lower limit value Vd.

The limiter signal Sa leads to the second comparator 20. The second comparator turns high when the periodic signal Sw from the input terminal 34 becomes greater than the limiter signal Sa, and turns low when the periodic signal Sw is smaller than the limiter signal Sa, thus the second comparator outputs the control signal Sc.

FIG. 3 is diagrams showing each signal when the temperature difference signal is between the lower limit Sd and the upper limit Su. That is, FIG. 3A shows the upper limit Su and the lower limit Sd, FIG. 3B shows the temperature difference signal Sn, FIG. 3C shows the limiter signal Sa, FIG. 3D shows the periodic signal and FIG. 3E shows the control signal Sc.

As shown in FIG. 3A and FIG. 3B, when the temperature difference signal Sn is between two limits, Sd and Su, the limiter signal Sa, which has a value Vn, is output from the limiter 18. The second comparator 20 compares, as shown in FIG. 3D, whether the value Vn of the limiter signal Sa is greater than the periodic signal Sw, and finally, the control signal Sc shown in FIG. 3E is generated.

FIG. 4 corresponds to the case when the temperature difference signal Sn is greater than the upper limit signal Su. FIGS. 4A to 4B show the upper limit signal Su and the lower limit signal Sd, the temperature difference signal Sn, the limiting signal Sa, the periodic signal with a triangular shape, and the control signal with a rectangular shape, respectively.

As shown in FIG. 4A and FIG. 4B, when the temperature difference signal Sn is greater than the upper limit signal Su, the limiter 18 passes the upper limit signal with the upper limit value Vu to the output. The second comparator 20 compares the periodic signal Sw with this limiting signal with the value Vu to generate the control signal Sc with a pulsed shape as shown in FIG. 4E.

FIG. 5 is the case when the temperature difference signal Sn is smaller than the lower limit signal Sd. FIG. 5A to 5E correspond to the upper and lower limit signals, Su and Sd, the temperature difference signal Sn, the limiting signal Sa, the periodic signal Sw with the triangular shape, and the control signal Sc with the pulsed shape, respectively.

When the temperature difference signal Sn is smaller than the lower limit signal Sd, the limiter 18 passes the lower limit signal Sd with the value Vd to the output as the limiting signal Sn. The second comparator 20 compares this limiting signal Sn with the periodic signal with the triangular shape, consequently, generates the control signal Sc with the pulsed shape.

Thus, the limiting signal Sa is restricted in its value between the lower limit Vd and the upper limit Vu. Moreover, the lower limit value Vd is greater than the negative maximum −Vw and the upper limit value Vu is smaller than the position maximum +Vw of the periodic signal Sw. Therefore, the duty ratio of the control signal does not saturate and become vacant. Here, the duty ratio is denoted as the period when the signal is high within one cycle of the signal.

The control signal Sc output from the second comparator 20 enters the PWM signal generator 12 that generates a plurality of PWM signals, Sg1 to Sg4. The duty ratio of the control signal Sc determines the duty ratio of the PWM signals, Sg1 to Sg4. These PWM signals, Sg1 to Sg4, enter the H-bridge 14.

FIG. 6 explains the operation of the H-bridge 14 and the Peltier device 4. The H-bridge 14 including two p-channel FETs, 41 and 42, and two n-channel FETs, 43 and 44, operates in response to the PWM signals, Sg1 to Sg4, as follows:

When the PWM signals, Sg1 and Sg3, turn high and the other PWM signals, Sg2 and Sg4, turn low, two FETs, 41 and 44, turn on and the other two FETs, 42 and 43, turn off. The current I₁ provided from the first FET 41 enters one terminal 4 a of the Peltier device 4 through the inductor 61, and is poured in the fourth FET 44 from the other terminal 4 b. On the other hand, when the PWM signals, Sg2 and Sg4, turn high, while the other PWM signals, Sg1 and Sg3, turn low, the FETs, 42 and 43, turn on and the other FETs, 41 and 44, turn off. The current I₂ provided from the FET 42 enters the other terminal 4 b of the Peltier device 4 through the inductor 62, and is sank in the third FET 43 from the terminal 4 a. Here, since the inductors, 61 and 62, and the capacitors, 51 and 52, form a filtering circuit, respectively, the currents, I₁ and I₂, which is a pulsed shape depending on the waveform of the control signal Sc, are filtered by these low pass filter to drive the Peltier device with an average of the pulsed currents, I₁ and I₂.

Accordingly, when the PWM signals, Sg1 to Sg4, have the duty ratio of 50%, the average current provided to the Peltier device can be cancelled, because the period the current flows from the first terminal 4 a to the second terminal 4 b becomes the same period the current flows from the second terminal 4 b to the first terminal 4 a. In this case, the Peltier device does not raise up nor cool down the temperature of the light-emitting device.

Moreover, the H-bridge mentioned above has a pair of two p-channel FETs and a pair of n-channel FETs. However, the H-bridge may be configured by four n-channels FETs or four p-channel FETs. In this case, the PWM signal Sg1 is provided to two FETs arranged in the upper left and in the bottom right, respectively, while the other PWM signal Sg2, complementary to the PWM signal Sg1, is provided to two FETs arranged in the upper right and in the bottom left, respectively, of the H-bridge.

Thus, since the PWM signals, Sg1 to Sg4, varies in their duty ratio depending on the monitored signal Sm and the target temperature signal St, the Peltier device 4 can be driven based on the practical temperature of the light-emitting device 2 and the target temperature thereof. Moreover, the limiting signal Sp limits the duty ratio of the PWM signals, Sg1 to Sg4, which enables the optical transmitter to control the Peltier device in forward without any resistor to sense the current flowing in the Peltier device. Accordingly, the power supply efficiency and the driving efficiency of the Peltier device can be enhanced.

Although the present invention thus described as referring to preferred embodiment, the invention is not restricted to those embodiments.

For example, in the embodiment above described, the limiter 18 receives the single limiting signal Sp, the receiver may receive a combination of limiting signals. In this case, the limiter generates the upper limit signal Su from the first limiting signal, while generates the lower limit signal Sd from the second limiting signal. The inverting amplifier 28 can be omitted.

Thus, the present optical transmitter provides a forward controlled Peltier device without any resistor to sense the current flowing in the Peltier device. 

1. An optical transmitter with a forward controlled Peltier device, comprising: a light-emitting device, a temperature of said light-emitting device being controlled by said Peltier device; a temperature sensor for monitoring a temperature of said light-emitting device and outputting a monitored signal; and a controller for controlling said temperature of said light-emitting device by receiving said monitored signal output from said temperature sensor, wherein said controller forwardly controls said temperature of said Peltier device without any sensing resistor connected in series to said Peltier device for sensing a current flowing in said Peltier device.
 2. The optical transmitter according to claim 1, wherein said controller includes a first comparator and a limiter, said first comparator comparing said monitored signal with a target signal corresponding to a target temperature of said light-emitting device and outputting a temperature difference signal denoting a temperature difference between said monitored temperature of said light-emitting device and said target temperature, and said limiter cramping said temperature difference signal between a preset maximum value and a preset minimum value.
 3. The optical transmitter according to claim 2, wherein said limiter outputs said preset maximum value when said temperature difference signal is greater than said preset maximum value and outputs said preset minimum value when said temperature difference signal is smaller than said preset minimum value.
 4. The optical transmitter according to claim 1, wherein said controller includes a second comparator for comparing said temperature difference signal with a periodic signal with a triangular shape having a positive peak value greater than said preset maximum value and a negative peak value smaller than said preset minimum value, and for outputting a pulse width modulation signal with a pulsed shape, said pulse width modulation signal being generated by slicing said periodic signal by said temperature difference signal.
 5. The optical transmitter according to claim 1, wherein said controller includes a switching ladder including a pair of two switches, each switches including a pair of switching devices operated in complementary, said Peltier device being connected between said pair of two switches through a low pass filter including an inductor and a capacitor such that a direction of current flowing in said Peltier device is altered by switching said pair of two switches in complementary.
 6. An optical transmitter having a semiconductor light-emitting device and an forward controlled Peltier device without any sensing resistor for monitoring an current flowing in said Peltier device, said Peltier device controlling a temperature of said light-emitting device, said optical transmitter comprising: a temperature sensor for monitoring said temperature of said light-emitting device, and for outputting a temperature-monitored signal; and a controller for controlling a current flowing in said Peltier device, said controller including, a first comparator for comparing said temperature-monitored signal with a target temperature signal denoting a target temperature for said light-emitting device, and for generating a temperature difference signal, a limiter for cramping said temperature difference signal between a preset maximum value and a preset minimum value, a second comparator for slicing a periodic signal by said temperature difference signal, said periodic signal having a triangular waveform with a positive peak value greater than said preset maximum value and a negative peak value smaller than said preset minimum value, and for outputting a control signal with a pulsed wave form, and a switching ladder including a pair of two switches each having two switching devices operating in complementary, said Peltier device being connected between said two switches through a low pass filter including an inductor and a capacitor, said switches being driven by said control signal and another control signal complementary to said control signal. 